X Rays to Go: Carbon nanotubes could shrink machines

Carbon nanotubes have been the darlings of the technology community for a decade. Researchers around the world have been touting them as promising components for making products ranging from microchips to medicines. Now, these tiny tubes have found their way into a novel X-ray machine that could improve examinations of patients in the hospital, victims at the scene of an automobile crash, or luggage at airport-security checkpoints.

X-RAY FIRST. This image of three sets of finger bones of a model hand was produced by a novel X-ray machine that uses nanotubes to generate high-energy electrons. Zhou et al./UNC and Applied Nanotechnologies

Unlike conventional machines, the new one doesn’t require high temperatures to generate high-energy electrons for producing X rays. A thin layer of carbon nanotubes operating at room temperature does the job, says developer Otto Zhou of the University of North Carolina at Chapel Hill.

“There’s been an enormous amount of hype about what carbon nanotubes can do, and very little actually has happened in terms of this kind of application,” comments Walter de Heer of the Georgia Institute of Technology in Atlanta. The X-ray method is “an application whose time has come,” he says.

A conventional X-ray machine generates electrons by heating metal filaments inside a vacuum chamber to temperatures as high as 2,000C. When those electrons hit another piece of metal, they produce X rays. The prototype devised by Zhou and his colleagues uses carbon nanotubes in place of the metal filaments. When exposed, unheated, to an electric field, the nanotubes behave like tiny electron guns.

Since metal filaments burn out easily at their high operating temperatures, the new devices will last longer, Zhou says. They should also save energy and time.

Moreover, because the prototype operates at room temperature, researchers will be able to develop very small machines for portable X-ray work, such as in an ambulance or airport-security and customs operations, says Zhou.

He and his colleagues describe the prototype machine in the July 8 Applied Physics Letters. The report includes X-ray images of a fish and of a medical model of a hand containing human bones.

“This important work . . . brings carbon-nanotube X-ray sources much closer to commercializable products,” comments Ray Baughman of the University of Texas at Dallas. X-ray sources based on carbon nanotubes might even be small enough someday for use in catheters inserted into the body, he says.

According to Zhou, he and his colleagues are working with manufacturers to put the first generation of carbon-nanotube X-ray machines on the market within 2 years.

A new family of technologies using similar methods might become possible. Says Baughman, “This X-ray application is one of the many exciting potential uses of carbon nanotubes as electron sources, from light-emitting flat-panel displays and high-intensity lamps to microwave generators and electrical-discharge tubes for electrical-surge protection.”

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